This invention relates to an arrangement for coupling a towed vehicle, e.g. a caravan, and a towing vehicle, e.g. a motor car.
The most commonly used coupling arrangement between a motor car and a trailer comprises a ball and socket joint, i.e. a spherical articulation, located well behind the rear axle of the car. These known coupling arrangements are convenient because the ball and socket joint, as a result of its universality, allows pitching motion about a horizontal transverse axis, allows rolling motion about a horizontal longitudinal axis and allows steering motion about a vertical axis. But the ball and socket joint also suffers from certain drawbacks.
In particular, the part of the trailer weight acting on the ball and socket joint (a) is largely borne by the rear axle of the car and (b) tends by leverage to lift the front end of the car thereby transferring weight from the front axle to the rear axle of the car. The rear axle can thus be easily overloaded, particularly with a full complement of passengers and a full trunk. Further, the handling of a motor car is generally adversely affected if the rear end of the motor car is depressed below the normal level running attitude as a result of extra loads imposed upon, or behind, its rear axle. Certain motor cars have "self-levelling" suspension systems whereby the rear of a motor car, when depressed, may be automatically raised to return the motor car to the correct, level, attitude. However, this refinement is not normally found on the cheaper makes of motor car. It operates by stiffening the sprung suspension system for the rear axle. But thus increasing the stiffness of the rear axle springing system does not however restore any weight that may have been lost from the front axle.
To overcome the above problem, the invention provides an arrangement for coupling a towed vehicle to a towing vehicle, wherein the pitching, rolling and steering movements that take place between the two vehicles are separately accommodated, and wherein said pitching movements are accommodated by a substantially horizontal, transverse hinge structure disposed nearest the towing vehicle, said hinge structure comprising first and second parts adapted to rotate relative to one another with said first part being positionally fixed relative to the towing vehicle and rotatably carrying the second part, motion between said two parts being transmitted by elastic means, means being provided for adjustably stressing the elastic means to bias said two parts into a desired angular relationship.
The elastic means, which may for instance comprise a rubber bush or a torsion bar, serve to resist motion between the two parts such as to provide a semblance of rigidity between the towing vehicle and the towed vehicle, thereby helping to distribute part of the added weight applied on the coupling to the front axle of the towing vehicle, and to the axle of the towed vehicle, while allowing for movements caused by road surface irregularities or driving onto or off ramps, and providing a sprung cushioning effect against shock loads.
The resulting improved weight distribution also enables the coupling to carry more weight. This makes it possible to move the center of gravity of the towed vehicle forwards or the axle of the latter backwards, and hence to overcome to an appreciable extent, it is believed, the problem of "snaking", i.e. the tendency for the towed vehicle to sway sideways and to follow a serpentine course.
By applying torsional force to the elastic means about the pitching movements axis the elastic means can be stressed in an adjustable manner to relieve the rear axle of the towing vehicle to a greater or lesser extent, the weight taken off the rear axle being shifted to the front axle of the towing vehicle and to the axle of the trailer.
Preferably, the second part comprises a hole for rotatably receiving a stub-shaft, carried by the towed vehicle, along the rolling movements axis to form a second hinge structure for accommodating said rolling movements, means being provided for releasably locking the stub-shaft in the hole against axial movement therein.
Preferably also, the stub-shaft is mounted at the front of the towed vehicle for angular motion about a third substantially vertical, hinge structure to accommodate said steering movements. In one form of construction, the stub-shaft comprises an eye pivotally mounted in a fork-like member.
By separately accommodating the pitching, rolling and steering movements that take place between a towed vehicle and a towing vehicle, by means of three separate hinge structures, it is then also possible to control the action of the three hinge structures separately.